The primary purpose of the project on fertility is to develop methods for identifying reproductive hazards. The majority of reproductive failure in women occurs before they know they are pregnant. We are currently focussing on two ways of assessing this reproductive loss. The first is development of questionnaire methods for studying fertility. We applied this method to an occupational cohort in a study of mercury vapor and nitrous oxide exposure in dental assistants. This study also allowed us to evaluate the accuracy of mail-questionnaire data on time to pregnancy, the basic data used to compare fertility among exposed and unexposed. The second approach is focussed on developing biological markers of impaired ovarian function. Estrogen and progesterone metabolites in daily urines are being measured to characterize follicular development, ovulation, and luteal function. Ultimately, we are working toward developing methods for studying reproductive hazards in small populations so that localized exposures can be evaluated.